PtCl2-Catalyzed Intramolecular Cyclization of α-Benzyl Allenoates to Afford Indenes and Furanones

Indene and furanone are important ring structures widely present in active pharmaceutical molecules. Here, we have developed a straightforward method for the synthesis of indene and furanone via PtCl₂-catalyzed intramolecular cyclization of α-benzyl allenoates. By altering the ester substitution pattern in the α-benzyl allenoates, we can regulate the reaction site, enabling two distinct intramolecular cyclization reactions that yield both indene and furanone products, respectively. For α-benzyl-substituted ethyl allenoate, the reaction proceeds via a 5-exo cyclization to form indene derivatives. In contrast, for α-benzyl-substituted tert-butyl allenoate, the reaction involves ester hydrolysis and intramolecular cyclization, yielding furanone products. This method operates efficiently under a 5 mol % PtCl₂ catalyst and exhibits good tolerance toward various functional groups. Furthermore, furanone products can be obtained on a gram scale and further smoothly converted into 1,2-disubstituted furan.